Heavy rain and possible thunderstorms have been forecast for parts of the UK, in a bad omen for weather this summer according to the legend of St Swithin’s Day.

The Met Office issued a yellow weather warning for rain between 8am and 11.59pm on Monday ahead of forecast downpours of up to 40mm over the course of a few hours.

It spans Wales and parts of western England – including Cardiff, Bangor, Liverpool and Exeter.

According to traditional folklore, the weather experienced on St Swithin’s Day (July 15) will continue for the next 40 days – meaning parts of the country could be in for a wet rest of the summer if superstition holds true.

The proverb of St Swithin says: “St Swithin’s Day if thou dost rain, for forty days it will remain, St Swithin’s Day if thou be fair, for forty days will rain na mair.”

The Met Office warned “torrential downpours” are likely, with 15-20mm of rainfall forecast in less than an hour in some places – and 30-40mm within three hours in others.

This may be accompanied by lightning throughout the day, the forecaster added.

There is a small chance some homes could be flooded or see power cuts and fast flowing or deep floodwater may also cause a danger to life.

Spray and flooding could lead to difficult driving conditions and road closures, the Met Office said.

There is also a chance some train and bus services will be delayed or cancelled in affected areas.

The heavy rain is expected to spread north across the affected area during the day.